IMPROVING FISH STOCK ASSESSMENTS by Committee on Fish Stock Assessment Methods

Author:Committee on Fish Stock Assessment Methods
Language: eng
Format: epub
Tags: Agriculture : Aquaculture and Fisheries
Publisher: NATIONAL ACADEMY PRESS
Published: 1998-02-27T00:00:00+00:00

4. The age of 50% maturity was much higher than the age of 50% selectivity to the fishery (Figure 5.1). The model created a population that could be quite susceptible to overexploitation because fish reproduced at a greater age than the age at which they are recruited to the fishery. Such a situation is exemplified by cod, haddock, and flounder in U.S. waters, which start to be recruited at age 1 and mature at age 2+, although the difference is more pronounced in the simulated populations.

5. The survey gear had a dome-shaped selectivity function as shown in Figure 5.1 ("survey selectivity"). This choice was made because dome-shaped selectivity and natural mortality are often confounded in stock assessment applications. In addition, one data set (3) had doubled survey catchability for the last 15 years. This feature mimicked a change in survey vessel; analysts were told that a change of vessel occurred after 15 years.

6. Most stock assessment models assume constant natural mortality. In the committee's simulations, natural mortality was constant for fish of all ages during a given year but varied from year to year; this is probably true in actual populations due to variations in predation by other species and the changing incidence of disease with age. Natural mortality was modeled as a uniform random variable between 0.18 and 0.27 (with a mean value of 0.225).

7. In some fisheries, catch statistics are inaccurate, which most likely involves underreporting (see Chapter 2). One data set (2) included underreporting of catch by 30%.

8. Various process and measurement errors in the population's dynamics and the data were included in the model for realism, including random variation in recruitment, fishery catchability, survey catchability, fishery selectivity, fishing effort, ageing error, and sampling for ages.

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